1. Field of the Invention
This invention relates to a process and apparatus for the production of aqueous ammonia by mixing anhydrous ammonia and water.
2. Background of the Art
This invention has particular application in the liquid fertilizer field in which anhydrous ammonia is mixed with water to produce aqueous ammonia having an ammonia concentration of from about 20 to about 30 percent by weight. In the production of aqueous ammonia, of this strength, considerable quantities of heat are evolved by heat of dilution and the aqueous ammonia solution should be maintained at as low a temperature as possible to avoid loss of ammonia during handling of the product, and to make possible the employment of low pressure apparatus.
The production of aqueous ammonia from water and anhydrous ammonia in large commercial plants encounters few cooling problems because an ample supply of cooling and refrigeration equipment is ordinarily available. However, in case a fertilizer dealer wishes to stock anhydrous ammonia separately and then produce aqueous ammonia as needed, the problem of refrigerating or cooling the aqueous ammonia is a real one. Ordinarily, in this latter situation, commercial plant-type refrigeration is not available, nor is the capital investment to construct it warranted. Therefore it would be very desirable to have an efficient process and apparatus for making aqueous ammonia from anhydrous ammonia which required only a minimum number of relatively simple operating steps and units.
In U.S. Pat. No. 2,890,937 a method for cooling an aqueous ammonia solution below its boiling point is disclosed which requires spraying water on the external surface of an aqueous ammonia storage tank. It should be noted that while this method is relatively simple, it is also a very inefficient method for cooling an aqueous ammonia solution because the ratio of surface area of the tank to the volume of the solution is much too low for efficient heat exchange through the storage tank wall.
In U.S. Pat. No. 2,825,630 a coolant is utilized to remove heat of dilution from a hot aqueous ammonia solution, by contacting the hot aqueous ammonia solution with a coolant in an indirect heat exchange step. The requirement of a coolant to remove the heat of dilution necessitates the maintenance and storage of a third material (the coolant) in addition to anhydrous ammonia and water. An efficient cooling process that eliminates the coolant would be simpler and therefor more desirable.
In U.S. Pat. No. 2,701,182 a process for preparing aqueous ammonia from anhydrous ammonia is disclosed which utilizes a multi-stage dilution of anhydrous ammonia with water wherein the initial pressure is sufficient to maintain the aqueous ammonia solution in a liquid form. A single stage dilution and the elimination of the initial pressure requirement of this process would be more desirable in that the number of operating steps and units would be decreased and less sophisticated equipment would be required.
In U.S. Pat. No. 3,295,927 a process is disclosed for the preparation of aqueous ammonia wherein the hot aqueous ammonia solution is heat exchanged with incoming process water to raise the temperature of the water prior to mixing with anhydrous ammonia. It is also disclosed that the hot aqueous ammonia solution may be heat exchanged, additionally, with anhydrous ammonia to raise the temperature of the anhydrous ammonia prior to mixing with the heated incoming water. Again, this requirement of additional operating steps and units defeats the purpose of providing a simple and efficient process for making aqueous ammonia.
U.S. Pat. No. 2,855,278 teaches a process for the preparation of aqueous ammonia from anhydrous ammonia. It is taught that a hot aqueous ammonia solution may be indirectly heat exchanged with anhydrous ammonia in a single cooling step; however, the process of this patent requires the multi-step dilution of anhydrous ammonia into water and thus would be more suitable for the large scale production of ammonia rather than the production of aqueous ammonia on a relatively small scale as needed by a fertilizer dealer.
U.S. Pat. No. 3,008,801 also teaches a multi-stage dilution process for the preparation of aqueous ammonia from anhydrous ammonia. In this process anhydrous ammonia, which has been heated by indirect heat exchange with a hot aqueous ammonia solution, is first diluted with water in a spray tower to provide an aqueous ammonia solution at about 200.degree. F. The hot aqueous ammonia solution is then sent through an air fin heat exchanger to cool such solution to about 160.degree. F., and then the cooled aqueous ammonia solution is divided and one part sent to the spray tower for a second dilution and the other part utilized to heat the anhydrous ammonia.
It is clear from the above that previous workers in the field have relied on multi-stage dilution or very sophisticated refrigeration and cooling equipment and procedures to remove the heat of dilution generated during the preparation of aqueous ammonia solutions having an ammonia concentration suitable for use as a liquid fertilizer.
Therefore, one object of the instant invention is to provide an improved process and apparatus for the production of aqueous ammonia at a strength suitable for use as a liquid fertilizer.
Another object is to provide an apparatus for forming aqueous ammonia which is of such simplicity that it can readily be made available in mobile form so that anhydrous ammonia can be converted into aqueous ammonia at any point at which the requisite constituents are available.
Another object is to reduce the cost of aqueous ammonia by reducing or eliminating expenses incident to the transportation of large volumes of water.
Another object is to eliminate the conventional requirement for an independent heat source to vaporize liquid anhydrous ammonia in conventional aqueous ammonia producing processes.
Another object is to eliminate the conventional requirement for elaborate refrigeration equipment in the production of aqueous ammonia involving the dilution of anhydrous ammonia into water.
Further objects and advantages are to provide improved steps, elements, and arrangements thereof in a process effective in accomplishing the intended purposes.
Other objects and advantages of the instant invention will become apparent from a careful reading of the specification below.